Nitrogen Metabolism Flashcards
What is an important feature about Amino acids when considering metabolism
Amino acids are not stored in the body for future use
How can Amino acids be obtained?
- The diet
- de novo synthesis
- Recovered from Protein degradation
Catabolism (break-down) of amino acids occurs in two stages what are they
- α-amino group removal through transamination - leaving ammonia which some will be lost in urine
- α-keto acids being converted to metabolic pathway intermediates (CO₂, H₂O, glucose, fatty acids + ketones)
The amino acid pool contains all free amino acids present within an organism (in cell, extracellular fluid, blood plasma)
This pool can have processes which add and take away from it - existing at equilibrium
What processes can add to it
- Amino acids from degraded body proteins (most)
- Amino acids from dietary proteins
- Synthesis of nonesseential amino acids from simple intermediates
What processes can deplete the amino acid pool
- Protein synthesis
- Nitrogen-containing molecule precursors
- Conversion to glucose, glycogen, fatty acids, ketone bodies etc
What enzyme degrades intraceullar proteins
- ATP-dependent ubiquitin-proteasome system
What enzymes degrade extracellular proteins
ATP-independent enzyme system of the lysosome
Proteins selected for degradation are…
Tagged with a molecule of ubiquitin
Occurs at a side chain of lysine, requiring ATP and enzymes to catalyse
Once a protein is ubiquitinated, what does this allow the proteasome to do
Allow cytosolic proteasome to unfold the protein, deubiquitinate it and transports the protein to the proteolytic core
Oligopeptides are formed (protein fragments) which are then acted on by general proteases and amino acids formed enter into the amino acid pool
Degradation of proteins is determined by which structural properties
- Oxidation levels of N-terminal residue
- PEST sequences (serine or aspartate at n-termine as they should be internal AA)
Once dietary proteins are consumed, they enter the stomach where they are broken down
How?
- The stomach excretes gastric juice (with HCL pH 2.5) which will denature proteins
- Pepsin is also produced, which is an endopeptidase, releasing AA and oligopeptides
Once food leaves the stomach, it nexts moves to the pancreas
Here, protein breakdown is said to be more specific, why?
- The pancreas produces proteases which are specific to an amino acids R group
- They are exo- and endoproteases
After the pancreas, food then moves to the small intestine, where…
exopeptidases (aminopeptidase) celave N-terminal residues to produce smaller peptides and AA
What are the ways that free amino acids and Di- and Tri-peptides enter into enterocytes
- Free amino acids are taken into enterocytes by the Na⁺ linked secondary transport system (using ATP to create gradient)
- Di- & Tripeptides are taken up by H⁺ linked transport system, where they are hydrolysed to AA and released into portal system
How do amino acids move into cells
- A concentration gradient exists between the outside of the cell and the inside, where amino acids are actively transported in
- 7 different transport system exist with overlapping specificities for amino acids
What occurs during a transamination reaction, in the breakdown of amino acids
The amino group is transferred to α-Ketoglutarate, forming Glutamate, using the enzyme Aminotraferases
Now we have an α-keto acid
What is the use of Glutamate in the cell once it is formed
Glutamate can be oxidatively deaminated or used as an amino donor group
This can be in non-essential amino acid synthesis
Aminotransferase are found in the liver, kidney and muscle
They are specific for amino group donors, e.g. a aminotransferase which formed alanine would be called…
alanine aminotransferase (ALT)
Aminotransferase are found in the liver, kidney and muscle
They are specific for amino group donors, e.g. a aminotransferase which formed alanine would be called…
alanine aminotransferase (ALT)
What is oxidative deamination
The reaction results in an amino group being released as ammonia from glutamate (which has taken it from the AA)
Occurs in the liver and kindey
Ammonia is then a source of nitrogen in hepatic urea synthesis
What does the enzyme Glutamate dehydrogenase do
This removes the amino group from glutamate and then forms ammonia
Triggered by lots of ADP as AA backbone can be transformed into glucose
Ammonia is highly toxic in the body, it is disposed as
Urea
It forms from an NH₃ from oxidative deamination and one from aspartate, as well as CO₂
This occurs in the liver, where it is transported to kidneys are excreted
The first two reactions of the Urea cycle occur?
In the mitocondrial matrix
The yellow blob
What happens in the 3rd step of the urea cycle
Citrulline reacts with Aspartate using the enzyme argininosuccinate synthetase and ATP to form Argininosuccinate
Fumerate is a TCA cycle intermediate
How can you form Malate, which is another TCA cycle intermediate
Using Fumarase
From here it will enter the mitocondria and the TCA cycle can continue
What happens in the 5th step of the Urea cycle
Arginine, reacts with Arginase which liberates Urea and forms Ornithine (we are back to the start)
What does Catabolism of Ketogenic Amino acids yield
Yields acetoacetate or acetyl Co-A or acetoacetyl Co-A
Carbon skeletons cannot lead to glucose
Once the electron are at ferrodoxin, what happens
through ATP hydrolysis MoFe protein can come closer (conformational change) to ferrodoxin and electrons can move onto it
This changes the MoFe proteins redox potential, allowing N₂ to be reduced
How can you form glutamine from glutamate
Glutamine synthetase can incorporate another ammonium ion into glutamate to form glutamine, driven by ATP
What is the difference between essential and non-esstential amino acids in terms of synthesis
Essential amino acids can only be obtained from the diet - due to complicated synthesis processes
Non-essential can be synthesised by mammals from common intermediates
For nonessential amino acids, many of them come from
glycolytic intermediates which are transaminated
